I just thought it would be nice to have so that way if the owner is away for awhile then who ever is second in charge can take care of the group.
I myself was not on here for awhile and some one had joined my group who then started to spam. Yes an admin was able to take care of the problem, but if some one else was in charge the problem could have been dealt with sooner. Plus because of this persons actions it forced me to set a permission for those who need to join.
I also agree with Canderis, this change would be beneficial for modding groups.